Introduction
Agrammatic aphasia is primarily characterized by (morpho)syntactic impairment in production, which is often selective not only across but also within different phenomena. Bastiaanse and colleagues, for example, consistently found past reference to be more impaired than present/future reference (e.g., Bastiaanse, 2008, 2013; Bastiaanse et al., 2011; Martínez-Ferreiro & Bastiaanse, 2013; Yarbay Duman & Bastiaanse, 2009). To account for this pattern, Bastiaanse et al. (2011) formulated the PAst DIscourse LInking Hypothesis (PADILIH), which posits that past reference is more demanding in terms of processing resources than present/future reference, because, unlike the latter, it involves discourse linking. Most of the evidence for PADILIH has been produced by using the Test for Assessing Reference of Time (TART) (Bastiaanse et al., 2008) in different languages. TART tests participants’ ability to “copy and paste” the tense feature from the source to the target sentence, and to retrieve the corresponding verb form/inflection. Therefore, it leaves unexplored the underlying ability to encode time reference-related abstract/prephonological features. Nevertheless, both encoding and retrieval abilities are involved in time reference (Faroqi-Shah & Thompson, 2007, and references therein).
The present study investigates the validity of PADILIH by considering both encoding and retrieval abilities. Within a cross-linguistic and cross-population approach, aphasic speakers and individuals with probable Alzheimer’s disease (AD) were tested. Since both populations are known to present processing limitations due to reduced verbal working memory capacity (e.g., Kensinger et al., 2003; Kok et al., 2007), PADILIH would predict the same pattern for both at the group level.
Methods
Twenty-nine individuals with agrammatic aphasia (10 Italians, 11 Greeks, 8 Germans), 29 individuals with mild-to-moderate AD (13 Italians, 16 Greeks) and 10-16 age- and education-matched controls for each (language-neurological) group participated in the study. Participants were administered a transformational sentence completion task (similar to that used by Friedmann and Grodzinsky, 1997) containing 20 items tapping past reference and 20 tapping future reference.
Results
All aphasic and AD groups fared significantly worse than control groups (in all comparisons, Fisher’s exact test, p.05) (Figure 1). Double dissociations emerged not only at the group but also at the individual level within all three aphasic groups and within the Italian AD group.
Discussion
The results do not lend cross-linguistic support to PADILIH, as only the data of the two Greek neurological groups were consistent with this hypothesis. The double dissociations within groups suggest that there are two different sources of difficulty for past reference and future reference that can differentially affect aphasic and AD speakers due to heterogeneity of associated deficits. Past reference may be impaired because it is discourse-linked and future reference because it refers to possible worlds, thus involving more abstract representations compared to past reference.

Figure 1

Acknowledgements

This research was supported by a Marie Curie Intra European Fellowship, awarded to the first author, within the 7th European Community Framework Programme.